Chemical reactions are the heart of chemistry. A chemical reaction is when one
or more substances are changing into other substances. Chemical reactions are
evidenced by the disappearance of characteristics of the starting substances and
the appearance of new properties that identify the products;

The law of conservation of mass states there is no loss of mass in the reaction process.

Reaction Equations

Reactions are identified
in chemistry using simple equations. An example reaction equation, of iron reacting with oxygen in the corrosion process, is provided below;

The reactants are shown on the left of the equation and the products are shown on the
right. The number of atoms
of each element will therefore be the same on both sides of the equation.

It is important first to define the components of a reaction.
It is also necessary to understand that several competing reactions may take place in a
given system, and their relative velocities (rates) will generally influence the
composition of the products.

Energy changes during reactions

A reaction can be exothermic (exoergic) or endothermic (endoergic) or for very rare cases athermic (aergic).

Exothermic reactions give out energy, usually as heat. The combustion reactions such as gas or wood burning, are examples of exothermic reactions.
Endothermic reactions take in energy from the surroundings. Reactions that
need a continuous supply of heat or electrical energy, such as thermal decomposition
reactions and electrolysis, are endothermic reactions. Athermic reactions involve no exchange of
energy

During the course of a reaction, chemical bonds are broken and new bonds formed.
The process of making bonds releases energy and the process of breaking bonds requires energy input.

A reaction process where more energy is evolved in bond making than is absorbed in the bond breaking
is exothermic. Conversely a reaction where more energy is absorbed in the bond breaking than
is evolved in the bond breaking is endothermic. The majority of spontaneous reactions are exothermic

In the process of the reaction the reactants become activated as they gain energy and it is only when
they reach a minimum activation energy (Ea) that the products are formed..

This activation energy value is a barrier to be overcome before the successful compeletion
of the reaction to the product stage. The figures below show the energy levels
throughout a typical reaction process. Ea1 represents the activation energy of
the forward direction and Ea2 is the activation energy of the reverse process..

Basic Types of Reactions

There are four basic types of reactions.

Substitution- involves the direct displacement of an atom(group) by another atom (group)

Addition- An unsaturated compound combines with another compound to produce one product

Elimination-Involves removal of atoms or groups of atoms from adjacent atoms to form multiple bonds or increase the degree of unsaturation in an existing unsaturated bond

Rearrangement-This is the migration of an atom or group of atoms from one site on the main
molecular skeleton to another site

This is typically a reaction where one chemical is oxidised
and another is reduced i.e one chemical gains oxygen and the other
loses oxygen. This definition has developed into a more general view
of the process in which oxidation is the loss of electrons
and reduction is the gain of electrons.
Ref. Oxidation - Reduction

Acid Base

A reaction between an acid and a base to form a salt and water as the only products.
A typical Acid- Base reaction is that between reactants Sulphuric acid and potassium
hydroxide resulting in products potassium sulphate and water i.e.